If you can run the program, use the Manage User Files option (under the Simulation menu) and delete it.
If the program won't start at all, manually delete all the associated files:
A subdirectory and a .BSE file from: C:\Users\USERNAME\Documents\RealFlight 7\Vehicles\CustomModels
A .COLORSCHEME file from: C:\Users\USERNAME\Documents\RealFlight 7\Vehicles\ColorSchemes
A .RFVEHICLE file from: C:\Users\USERNAME\Documents\RealFlight 7\Vehicles\CustomVehicles